Apparatus for and methods of bending metal strip

Description

[0001] This invention is concerned with an apparatus for bending metal strip comprising guide means . along which a metal strip is fed by feed means and having an exit end from which the metal strip fed as aforesaid projects, a bending tool spaced from the exit end of the guide means and rotatable about an axis which extends heightwise of the strip and is disposed at the exit end of the guide means, whereby the projecting portion of the metal strip can be engaged by the bending tool and be bent thereby about said exit end, and computer control means for controlling the amount of feed of the metal strip along the guide means and also the amount of rotation of the bending tool.

[0002] One such apparatus is disclosed in EP-A 88576, which document falls under the scope of Art 54 (3) EPC, said apparatus being. especially, but not exclusively, for use in the manufacture of press knives for cutting out leather blanks from which boots and shoes can subsequently be made.

[0003] One problem which arises in using this apparatus is caused by the fact that, when bending metal strip for e.g. knife manufacture, the strip is already hardened and tempered and exhibits a significant springback characteristic, so that after bending has taken place and the strip is released, there is a tendency for it to return part-way to its unbent condition, the degree of such springback varying of course from strip to strip.

[0004] In US-A 3352136 is disclosed a pipe bending machine which addresses the problem of springback. In this machine a springback detector is provided in the form of two sensing fingers arranged to engage the pipe after bending. The detector is mounted on the bending tool, but is maintained in an out-of-the-way position during the bending operation. For an initial, datum, setting the sensing fingers are aligned with the front face of a clamp by which the pipe is held during the bending operation; in this position the fingers would engage the edge of the pipe while clamped.

[0005] For detecting springback, the pipe is released from the clamp and moves part-way back to its original condition, whereafter the detector is brought into engagement therewith, the sensing fingers thus being rotated through an amount which is proportional to the degree of springback. A monitor is provided for monitoring this amount of rotation which is representative of the springback characteristic, the information thus acquired being stored in the computer memory. This information can then be used for modifying the bending data in order to achieve a desired angle of permanent deformation.

[0006] It will be appreciated that such an arrangement would be expensive not only in so far as additional detecting parts have to be provided, but also by virtue of the relative complexity of those parts.

[0007] Also in DE-A 1920265 is disclosed a bending machine, in this case for metal rods, wherein overbending can be pre-set in order to accommodate the springback characteristic of the rods. In this case, it is a question of setting the position of switching elements by which the bending operation is terminated, the setting of the elements taking into account not only the desired angle of permanent deformation, but also the additional angle through which the material has to be bent in order to achieve the desired angle of permanent of deformation. In this specification, however, there is no reference to the use of the machine for detecting springback, nor how the additional angle is calculated.

[0008] It is the object of the present invention to provide an improved apparatus for bending metal strip wherein a simple and reliable springback detection system is provided which utilises the bending equipment and its associated controls.

[0009] The invention thus provides apparatus for bending metal strip comprising guide means along which a metal strip is fed by feed means and having an exit end from which the metal strip fed as aforesaid projects, and a bending tool spaced from the exit end of the guide means and rotatable about an axis which extends heightwise of the strip and is disposed at the exit end of the guide means, whereby the projecting portion of the metal strip can be engaged by the bending tool and be bent thereby about said exit end, wherein, for sensing the springback characteristic of the strip, the bending tool and the metal strip are electrically insulated from one another, a potential being applied to the tool, which potential, upon physical contact taking place between the bending tool and the strip, becomes zero, thereby indicating that contact has occurred, the apparatus also comprising computer control means for controlling the amount of feed of the metal strip along the guide means and also the amount of rotation of the bending tool, said computer control means also serving to monitor the potential on the bending tool and also the amount of rotation effected by the bending tool into physical contact with the strip as aforesaid, as indicated by the change in the potential on the tool, such rotational amount being representative of the permanent angular deformation of the strip.

[0010] It will thus be appreciated that in this way simply by electrically insulating the bending tool and by the provision of appropriate software the springback characteristic of a piece of metal strip can readily be detected.

[0011] In accordance with the present invention, furthermore, preferably the bending tool is mounted in a metal block which is rotatable about said axis, said block being mounted in the apparatus by means of an electrically insulating bearing. More particularly, conveniently the bending tool is movable relative to the block heightwise of the strip into and out of an operative condition in which it can engage the projection portion of metal strip to bend it as aforesaid, such movement being effected by a piston-and-cylinder arrangement acting on the bending tool, and the metal block is electrically insulated from the piston-and-cylinder arrangement by an electrically insulating member, while the piston, which is engaged by the bending tool direct, is of an electrically insulating material.

[0012] It will thus be appreciated that, in accordance with the present invention, a relatively simple arrangement is provided for detecting springback, which utilises the bending equipment and associated controls, with the need only for electrically insulating the bending tool from other parts of the apparatus to achieve this purpose.

[0013] The invention is also concerned with a method of compensating for springback when bending metal strip, using an apparatus as set out above. In accordance with the present invention, therefore, the method is characterised by the steps of (i) supporting a length of metal strip with a leading end portion thereof in a projecting, unclamped, condition, (ii) moving a bending tool against the projecting end portion of the metal strip through a predetermined distance to create a bend of a predetermined angle in said end portion, said bend being disposed at the boundary between the supported and projecting portions of the strip, (iii) retracting the bending tool out of engagement with the strip, (iv) moving the bending tool again into engagement with the now bent projecting end portion of the strip, (v) monitoring . the amount of such movement, and thus the angle of permanent deformation of the strip, (vi) comparing said amount of such movement with the predetermined distance, (vii) repeating steps (ii) to (vi) for a plurality of different predetermined distances, (viii) determining from the data thus obtained the springback characteristic of the metal strip, and (ix) using the characteristic thus obtained to calculate the distance through which the bending tool is to be moved to obtain a bend having a desired angle of permanent deformation.

[0014] It will thus be appreciated that, by using the computer control means of the apparatus, it is necessary only to provide details of the angles of permanent deformation together with an identification of the metal strip to be used, and the computer control means will calculate the necessary modifications and effect the appropriate bending operations.

[0015] In accordance with the present invention, preferably the springback characteristic is a polynomial with variables selected to give a curve passing through of close to the points found by .movement of the bending tool in accordance with step (v), this characteristic then being stored in the computer control means of the apparatus, which means is also used for carrying out steps (viii) and (ix) and thereafter supplying control signals to the bending tool appropriate to the bending operations required.

[0016] Furthermore, it is preferable that the apparatus be initially calibrated. To this end, conveniently the bending tool is successively moved against a plurality of portions of strip each of which has been bent to a known angle of permanent deformation and such movement is monitored to derive appropriate data relating movement of the bending tool to angular deformation of the strip.

[0017] It will be appreciated that, in using the apparatus in accordance with the invention, in general a series of bends will be provided in the strip, the strip being fed intermittently between successive bending operations. In some instances, furthermore, the increment of feed, i.e. the distance between adjacent bends, will be less than the distance by which the bending tool is spaced from the boundary between the supported and projecting portions of the metal strip. In such a case, in accordance with the present invention, conveniently the computer control means, in carrying out step (ix), also calculates the amount by which the strip is offset from the plane in which it is supported and modifies the data accordingly. This calculation is referred to as "offset compensation".

[0018] The above and other features of the invention will become clearer from the following detailed description, to be read with reference to the accompanying drawings, of one apparatus and one method in accordance with the invention. It will of course be appreciated that this apparatus and this method have been selected for description merely by way of non-limiting example of the invention.

[0019] In the accompanying drawings:

Fig. 1 is a fragmentary plan view showing an operating locality of the apparatus in accordance with a preferred embodiment of the invention;

Fig. 2 is a side view, essentially diagrammatic, of a bending head of the apparatus shown in Fig. 1;

Fig. 3 is a view generally similar to Fig. 1, but showing a metal strip in the process of being bent, and illustrating the difference between the degree of bending and the angle of permanent deformation in the strip;

Fig. 4 is a view similar to Fig. 3 but illustrating how the apparatus is effective in the case of offset compensation (as hereinbefore' defined);

Figs. 5 and 6 are graphs showing characteristics which are derived empirically and subsequently stored in a microcomputer constituting computer control means of the apparatus; and

Fig. 7 is a flow chart indicating the modification of the input data to take account of the springback characteristic and offset compensation.

[0020] The apparatus now to be described is a metal strip bending apparatus which comprises guide means 12 including a slot 10 along which metal strip is fed by means of longitudinal feed means (not shown), and in which the strip is clamped against longitudinal movement during a bending operation. The exit end 14 of the slot 10 is coincident with a vertical pivot axis X of a generally cylindrical bending head 16 comprising a bending tool in the form of a pin 18. The axis of the pin 18 is spaced from the pivot axis X by a distance which is conveniently between 6 and 10 mm, preferably 8.5 mm. The bending head 16 is accommodated in a table 20 and, in its operative position, i.e. its position for bending the strip, the pin 18 projects above the table surface (Fig. 2). At its upper end of the pin 18, is provided with a flange or shoulder 22 by which it engages the strip in a central region thereof.

[0021] The pin 18 is mounted for vertical sliding movement within a metal block 22 of the bending head 16. The block 22 has a cylindrical body 24 and two radially projecting lugs 26 which fit within opposed radial slots formed in an upper end of a wall of a brass cylinder 28, forming part of a pneumatic piston-and-cylinder arrangement. The brass cylinder 28 is rotatable about the axis X by means of an actuator (not shown) to cause the pin 18 to undergo controlled lateral movement in relation to, and thereby engage and bend, the strip about the axis X. The lower end of the pin 18 extends through a metal plug 30 and rests against a recessed upper end of a piston 32 slidable in a chamber 34 forming part of said pneumatic piston-and-cylinder arrangement. Air under pressure admitted to the chamber 34 forces the piston 32 upwards and thereby moves the pin 18 to its operative position. Two return springs, indicated diagrammatically at 36, act between the underside of the block 22 and the piston 32 to move the pin 18 to a retracted position, in which the upper end of the pin can pass beneath the strip, to the other side thereof, for bending in the opposite sense.

[0022] The apparatus also comprises computer control means by which the angle of rotation of the bending head 16, the retraction and extension of the pin 18 and the longitudinal movement of the strip are controlled, whereby to effect a succession of bends in the strip to impart thereto the desired profile.

[0023] The pin 18 is electrically insulated from the strip and from the remainder of the apparatus by electrically insulating components shown shaded in Fig. 2. A plastics bearing 38 insulates the block 22 from the surrounding portions of the table 20 and the lugs 26 are electrically insulated from the brass cylinder 28 by a plastics film 40; the piston 32 is made of an electrically insulating material. The pin 18 is maintained at a potential, e.g. 5 volts, through an electrical connection made to the block 22. When the pin 18 contacts the strip, however, the pin 18 is earthed and the potential on the pin 18 becomes zero, so that sensing of this voltage gives an indication of when the pin and strip are in contact.

[0024] In Fig. 3 the metal strip 42 is shown being bent. After applying each bend to the strip by rotation of the bending head 16, the strip 42 tends to spring back. To inspect or check the permanent angular deformation of the strip 42 the head 16, after having been rotated out of engagement with the strip, is rotated once more so that the pin 18 engages the previously bent strip. The electrical potential of the pin 18 is monitored and when this potential becomes zero, thereby indicating contact with the strip, the microcomputer notes the angle A1 through which the head 16 has been rotated from a datum position in which its axis lies in the plane of the strip supported in the slot 10 (see Fig. 3). From the angle A1 the microcomputer derives the permanent angular deformation A2 from the characteristic shown in Fig. 5.

[0025] The characteristic shown in Fig. 5 is derived in an initial calibration step when the apparatus is first set up. Metal strips having known angles of bend A2 are placed in the apparatus and the bending head 16 is rotated until the pin 18 makes electrical contact with the strip. As before, contact is detected by the potential of the pin 18 becoming zero. The microcomputer stores the set of angles A1 corresponding to the known angles A2 and fits a sixth order polynomial through the points to give the resulting characteristic shown in Fig. 5.

[0026] The apparatus in accordance with the invention compensates for the springback of the strip after bending by means of a previously derived characteristic relating displacement of the bending head 16 to the resulting permanent angle of deformation applied to the strip. This characteristic is derived, in carrying out the method in accordance with the invention, by displacing the bending head 16 through a series of known magnitudes of displacement (angles of rotation A4) and causing a series of bends to be introduced into the strip, allowing the strip to springback after each bend, and detecting the permanent angular deformation (bend angles A5) in the strip after each bend by moving the pin 18 into contact with the permanently deformed strip, sensing when the tool contacts the strip and deriving therefrom a springback characteristic relating the permanent angular deformation to its appropriate known magnitude of displacement. The permanent bend angles A5 are of course derived from the characteristic shown in Fig. 5. The characteristic shown in Fig. 6, viz. the springback characteristic, thus relates actual rotation A4 to the deflection A5 of the strip. A sixth order polynomial is fitted through the points to give the springback characteristic which is held in the microcomputer.

[0027] This characteristic is derived as a preliminary step before each batch of strip is bent and may be regarded as calibration for the characteristics of the metal strip batch. A number of such characteristics may be stored for different strips and any characteristic may be modified to take account of any variations.

[0028] Fig. 4 illustrates an example of a situation in which offset compensation is provided for added accuracy of the bend applied to the strip 42. Where an increment of longitudinal feed of the strip 42 is less than the spacing of the axis of the pin 18 from the axis X, the pin 18 will not engage the strip in the usual manner (see the dotted line in Fig. 4), i.e. where the microcomputer would expect the pin 18 to make contact with the strip, but rather moves beyond the plane of the slot 10 (see solid line in Fig. 4) to make such contact. The angle A3 which must be corrected is the offset angle and the magnitude of this angle in any given situation will depend on the number and magnitude of the bends between the contact position of the pin 18 and the exit end 14 of the slot 10. The offset angle can be calculated trigonometrically and this is done by the microcomputer using data relating to previous bends. This calculation and the application of the compensation is referred to herein as "offset compensation".

[0029] Fig. 7 is a flow chart illustrating how springback compensation and offset compensation are applied in practice. The data generated by the microcomputer is in the form of a series of instructions consisting of longitudinal feed instructions (1) and bend angles (0). This data 46 is modified in the springback compensation stage 48 which alters the instructed bend angles in accordance with the characteristic of Fig. 6. Decision block 50 governs whether offset compensation is necessary. If not, the data proceeds to stage 52. Should offset compensation be necessary, the microcomputer computes the magnitude and direction in stage 54 and adds this to the bend angle in stage 56 to give a final bend instruction which is used to control the rotation of the bending head 16.

Claims

1. Apparatus for bending metal strip comprising guide means (12) along which a metal strip is fed by feed means (-) and having an exit end from which the metal strip fed as aforesaid projects, and

a bending tool (18) spaced from the exit end of the guide means (12) and rotatable about an axis which extends heightwise of the strip and is disposed at the exit end of the guide means (12), whereby the projecting portion of the metal strip can be engaged by the bending tool (18) and be bent thereby about said exit end,

wherein, for sensing the springback characteristic of the strip, the bending tool (18) and the metal strip are electrically insulated from one another, a potential being applied to the tool (18), which potential, upon physical contact taking place between the bending tool (18) and the strip, becomes zero, thereby indicating that contact has occurred,

the apparatus also comprising computer control means for controlling the amount of feed of the metal strip along the guide means (12) and also the amount of rotation of the bending tool (18), said computer control means also serving to monitor the potential on the bending tool (18) and also the amount of rotation effected by the bending tool (18) into physical contact with the strip as aforesaid, as indicated by the change in the potential on the tool (18), such rotational amount being representative of the permanent angular deformation of the strip.

2. Apparatus according to Claim 1 wherein the bending tool (18) is mounted in a metal block (22) which is rotatable about said axis, and the metal block (22) is mounted in the apparatus by means of an electrically insulating bearing (38).

3. Apparatus according to Claim 2 wherein the bending tool (18) is movable relative to the block (22) heightwise of the strip into and out of an operative condition in which it can engage the projecting portion of metal strip to bend it as aforesaid, such movement being effected by a piston-and-cylinder arrangement (28, 32) acting on the bending tool (18), and the metal block (22) is electrically insulated from the piston-and-cylinder arrangement (28, 32) by an electrically insulating member (40), and the piston (32), which is engaged by the bending tool (18), is of an electrically insulating material.

4. Method of compensating for springback when bending metal strip using an apparatus according to any one of the preceding Claims, said method comprising the steps of

(i) supporting a length of metal strip with a leading end portion thereof in a projecting, undamped, condition,

(ii) moving a bending tool against the projecting end portion of the metal strip through a predetermined distance to create a bend of a predetermined angle in said end portion, said bend being disposed at the boundary between the supported and projecting portions of the strip,

(iii) retracting the bending tool out of engagement with the strip,

(iv) moving the bending tool again into engagement with the now bent projecting end portion of the strip,

(v) determining the amount of such movement, and thus the angle of permanent deformation of the strip,

(vi) comparing said amount of such movement with the predetermined distance,

(vii) repeating steps (ii) to (vi) for a plurality of different predetermined distances,

(viii) determining from the data thus obtained the springback characteristic of the metal strip, and

(ix) using the characteristic thus obtained to calculate the distance through which the bending tool is to be moved to obtain a bend having a desired angle of permanent deformation.

5. Method according to Claim 4 characterised in that, as an initial calibration, the bending tool is successively moved against a plurality of portions of strip each of which has been bent to a known angle of permanent deformation and such movement is determined to derive appropriate data relating movement of the bending tool to angular deformation of the strip.

6. Method according to either one of Claims 4 and 5 characterised in that said characteristic is a polynomial with variables selected to give a curve passing through or close to the points found by movement of the bending tool in accordance with step (v), and in that the characteristic is stored in the computer control means of the apparatus, which means is also used for carrying out steps (viii) and (ix) and thereafter supplying control signals to the bending tool appropriate to the bending operations required.

7. Method according to Claim 6 wherein a series of bends is provided in the strip, the strip being fed intermittently between successive bending operations, characterised in that, where the increment of feed is less than the distance by which the bending tool is spaced from the boundary between the supported and projecting portions of the strip, the computer control means, in carrying out step (ix), also calculates the amount by which the strip is offset from the plane in which it is supported and modifies the data accordingly.

Ansprüche

1. Vorrichtung zum Biegen von Metallband mit einer Führungseinrichtung (12), längs welcher ein Metallband mittels einer Zuführeinrichtung zugeführt wird und welche ein Austrittsende aufweist, aus welchem das zugeführte Metallband herausragt, wobei ein Biegewerkzeug (18) von dem Austrittsende der Führungseinrichtung (12) im Abstand angeordnet ist und um eine Achse drehbar ist, die sich über die Breite des Bandes erstreckt, wobei das Biegewerkzeug an dem Austrittsende der Führungseinrichtung (12) vorgesehen ist und der vorspringende Abschnitt des Metallbandes von dem Biegewerkzeug (18) ergreifbar und dann um das Austrittsende abbiegbar ist, wobei zum Abfühlen der Rückfederungseigenschaften des Bandes das Biegewerkzeug (18) und das Metallband voneinander elektrisch isoliert sind und an das Werkzeug (18) ein Potential anlegbar ist, welches bei physikalischem Kontakt zwischen dem Biegewerkzeug (18) und. dem Band Null wird, so daß das Eintreten eines Kontaktes angezeigt wird, wobei eine Computer- steuereinrichtung zum Steuern der Zuführung des Metallbandes längs der Führungseinrichtung (12) und zum Steuern der Rotation des Biegewerkzeuges (18) vorgesehen ist, wobei die Computersteuereinrichtung ebenfalls zum Überwachen des Potentials an dem Biegewerkzeug (18) und der von dem Biegewerkzeug (18) bewerkstelligten Rotation in physikalischen Kontakt mit dem Band dient, der von der Änderung des Potentials an dem Werkzeug (18) angezeigt wird, wobei diese Rotationsgröße repräsentativ für die permanente winkelförmige Verformung des Bandes ist.

2. Vorrichtung nach Anspruch 1, dadurch gekennzeichnet, daß das Biegewerkzeug (18) in einem Metallblock (22) angeordnet ist, welcher um diese Achse drehbar ist, und daß der Metallblock in der Vorrichtung mit Hilfe eines elektrisch isolierenden Lagers (38) angeordnet ist.

3. Vorrichtung nach Anspruch 2, dadurch gekennzeichnet, daß das Biegewerkzeug (18) in Bezug auf den Block (22) über die Breite des Bandes in und aus einer Betriebslage bewegbar ist, in welcher es den vorspringenden Abschnitt des Metallbandes zwecks Abbiegens ergreifen kann, daß diese Bewegung von einer Kolben-Zylinder-Anordnung (28, 32) bewirkt ist, welche auf das Biegewerkzeug (18) einwirkt, daß der Metallblock (22) von der Kolben-Zylinder-Anordnung (28, 32) mittels eines elektrisch isolierenden Bauteiles (40) elektrisch isoliert ist, und daß der Kolben (32), welcher mit dem Biegewerkzeug (18) in Eingriff steht, aus einem elektrisch isolierenden Material besteht.

4. Verfahren zum Kompensieren des Rückfederns beim Biegen eines Metallbandes unter Verwendung der Vorrichtung nach einem der vorhergehenden Ansprüche, wobei

(i) eine Länge eines Metallbandes getragen wird, wobei ein führender Endabschnitt des Bandes vorspringt und nicht festgehalten wird,

(ii) ein Biegewerkzeug gegen den um eine vorbestimmte Strecke vorspringenden Endabschnitt des Metallbändes bewegt wird, um ein Biegen dieses Endabschnittes in einem vorbestimmten Winkel zu schaffen, wobei das Abbiegen an der Grenze zwischen dem gestützten und dem vorspringenden Abschnitt des Bandes erfolgt,

(iii) das Biegewerkzeug aus dem Eingriff mit dem Band zurückgezogen wird,

(iv) das Biegewerkzeug wiederum mit dem nun abgebogenen vorspringenden Endabschnitt des Bandes in Eingriff bewegt wird,

(v) die Größe einer solchen Bewegung und somit der Winkel der permanenten Verformung des Bandes bestimmt wird,

(vi) diese Größe der Bewegung mit der vorbestimmten Strecke verglichen wird,

(vii) die Stufen (ii) bis (vi) für eine Vielzahl von unterschiedlichen vorbestimmten Strecken wiederholt werden,

(viii) aus den so erhaltenen Daten die Rückfederungseigenschaften" des Metallbandes bestimmt werden und

(ix) die so erhaltenen Eigenschaften verwendet werden, um die Strecke zu berechnen, über welche das Biegewerkzeug bewegt werden muß, um eine Abbiegung mit einem gewünschten Winkel permanenter Verformung zu erhalten.

5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß als anfängliches Eichen das Biegewerkzeug aufeinanderfolgend gegen eine Vielzahl von Abschnitten des Bandes bewegt wird, die je zu einem bekannten Winkel permanenter Verformung abgebogen sind, und daß diese Bewegung bestimmt wird, um zweckmäßige Daten in Bezug auf die Bewegung des Biegewerkzeuges zwecks winkelmäßiger Verformung des Bandes abzuleiten.

6. Verfahren nach einem der Ansprüche 4 bis 5, dadurch gekennzeichnet, daß dieses Charakteristikum ein Polynom mit Variablen ist, das ausgewählt ist, um eine Kurve zu ergeben, die sich durch oder nahe der Punkte erstreckt, die durch Bewegen des Biegewerkzeuges gemäß Stufe (v) gefunden wurden, und daß dieses Charakteristikum in der Computer-Steuereinrichtung der Vorrichtung gespeichert wird, welche ebenfalls dazu verwendet wird, um die Stufen (viii) und (ix) auszuführen und welche danach Steuersignale zum Biegewerkzeug liefert, die für die erforderlichen Biegevorgänge zweckmäßig sind.

7. Verfahren nach Anspruch 6, wobei eine Reihe von Abbiegungen des Bandes erfolgen und das Band zwischen aufeinanderfolgenden Biegevorgängen intermittierend zugeführt wird, dadurch gekennzeichnet, daß dann, wenn die Zuführgröße geringer als der Abstand ist, in dem sich das Biegewerkzeug von der Grenze zwischen dem gestützten und vorspringenden Abschnitt des Bandes befindet, die Computer-Steuereinrichtung bei Ausführung der Stufe (ix) ebenfalls den Betrag berechnet, um welchen das Band aus der Ebene versetzt ist, in welcher es gestützt ist, wobei die Daten entsprechend modifiziert werden.

Revendications

1. Machine pour cintrer les bandes métalliques comprenant

des moyens de guidage (12) le long desquels une bande métallique est avancée par des moyens d'avance et qui présentent une extrémité de sortie au-delà de laquelle la bande métallique avancée comme indiqué précédemment fait saillie, et

un outil de cintrage (18) espacé de l'extrémité de sortie des moyens de guidage (12) et pouvant tourner autour d'un axe qui s'étend selon la hauteur de la bande et est disposé à l'extrémité de sortie des moyens de guidage, de sorte que la partie en saillie de la bande métallique peut être attaquée par l'outil de cintrage (18) et être ainsi cintrée autour de ladite extrémité de sortie,

dans laquelle, pour détecter la caractéristique de retour élastique de la bande, l'outil de cintrage (18) et la bande métallique sont isolés électriquement l'un de l'autre,'un potentiel étant appliqué à l'outil (18), lequel potentiel s'annule au moment où il se produit un contact physique entre l'outil de cintrage (18) et la bande, en indiquant par ce moyen qu'il s'est produit un contact,

la machine comprenant en outre des moyens de commande par ordinateur destinés à commander l'amplitude d'avance de la bande métallique le long des moyens de guidage (12) et aussi l'amplitude de la rotation de l'outil de cintrage (18), lesdits moyens de commande par ordinateur servant en outre à contrôler le potentiel de l'outil de cintrage (18) et aussi l'amplitude de la rotation exécutée par l'outil de cintrage (18) pour entrer en contact physique avec la bande comme décrit précédement, ce qui est indiqué par la variation du potentiel sur l'outil (18), cette amplitude de rotation étant représentative de la déformation angulaire permanente de la bande.

2. Machine selon la revendication 1, dans laquelle l'outil de cintrage (18) est monté dans un bloc métallique (22) qu'on peut faire tourner autour dudit axe, et le bloc métallique (22) est monté dans la machine à l'aide d'un palier électriquement isolant (38).

3. Machine selon la revendication 2, dans laquelle l'outil de cintrage (18) est mobile par rapport au bloc (22) dans le sens de la hauteur de la bande pour venir se placer dans une position de travail dans laquelle il peut entrer en contact avec la partie en saillie de la bande métallique pour la cintrer comme indiqué précédement et pour s'écarter de cette position de travail, ce déplacement étant effectué par un dispositif à piston et cylindre (28, 32) qui agit sur l'outil de cintrage (18) et la bloc métallique (22) est isolé électriquement du dispositif à piston et cylindre (28,32) par un élément électriquement isolant (40), tandis que le piston (32), qui est directement en prise avec l'outil de cintrage (18) est fait d'une matière électriquement isolante.

4. Procédé de compensation du retour élastique dans le cintrage d'une bande métallique au moyen d'une machine selon l'une quelconque des revendications précédentes, ce procédé comprenant les phases qui consistent à:

(i) supporter une longueur de bande métallique avec une partie d'extrémité de tête de cette bande qui fait saillie et est non serrée,

(ii) déplacer un outil de cintrage disposé contre la partie d'extrémité en saillie de la bande métallique, sur une distance prédéterminée, pour créer une courbure d'un angle prédéterminé dans cette partie d'extrémité, cette courbure étant située à la limite entre la partie supportée et la partie en saillie de la bande,

(iii) rétracter l'outil de cintrage hors de contact avec la bande,

(iv) ramener l'outil de cintrage en contact avec la partie en saillie de la bande, qui est maintenant cintrée,

(v) controler l'amplitude de ce mouvement et, par conséquent, l'angle de déformation permanente de la bande,

(vi) comparer l'amplitude de ce mouvement à la distance prédéterminée,

(vii) répéter les phases ii) à VI) pour plusieurs distances différentes prédéterminées,

(viii) déterminer, à partir des données ainsi obtenues, la caractéristique de retour élastique de la bande métallique, et

(ix) utiliser la caractéristique ainsi obtenue pour calculer la distance sur laquelle on doit déplacer l'outil de cintrage pour obtenir une courbure ayant un angle de déformation permanente désiré.

5. Procédé selon la revendication 4, caractérisé en ce que, à titre d'étalonnage initial, on déplace l'outil de cintrage successivement contre plusieurs parties de la bande dont chacune a été cintrée à un angle de déformation permanente connu et on détermine ce déplacement pour en dériver une donnée appropriée qui établit la relation entre le déplacement de l'outil de cintrage et la déformation angulaire de la bande.

6. Procédé selon l'une quelconque des revendications 4 et 5, caractérisé en ce que la caractéristique de retour élastique est un polynôme comprenant des variables sélectionnées pour donner une courbe qui passe par les points trouvés par le déplacement de l'outil de cintrage selon la phase (v), ou à proximité de ces points, cette caractéristique étant ensuite mémorisée dans les moyens de commande par ordinateur de la machine, ces moyens étant aussi utilisés pour effectuer les phases (vii) et (ix) et, ensuite, fournir à l'outil de cintrage des signaux de commande qui sont appropriés pour les opérations de cintrage voulues.

7. Procédé selon la revendication 6, dans lequel on forme une série de courbures dans la bande, la bande étant avancée par intermittence entre les opérations de cintrage successives, caractérisé en ce que, lorsque l'incrément d'avance est inférieur à la distance dont l'outil de cintrage est espacé de la limite entre la partie supportée et la partie en saillie de la bande, les moyens de commande par ordinateur, dans l'exécution de la phase (ix) calculent aussi l'amplitude du décalage de la bande au plan dans lequel elle est supportée et modifient la donnée en conséquence.

Drawing